1. Field of the Invention:
This invention relates to a variable capacitance diode device suited to tuning with an electronic tuning circuit.
2. Description of the Prior Art:
Variable-capacitance diode devices have recently found extensive use as tuning elements for electronic tuning circuit. There have conventionally been demands for variable-capacitance diode devices with a wide voltage range. With a variable-capacitance diode device, as the potential difference across the junction thereof increases with a reverse bias voltage (referred to as applied voltage hereinafter), the width of depletion layer tends to expand with a distribution of impurity concentration in the semiconductor layer. To achieve a variable-capacitance diode device with a wide voltage range, it has been the usual practice that the impurity concentration in that region of the semiconductor layer where the depletion layer tends to expand, is controlled so as to be proximate to the Gaussian distribution, i.e., so as to decrease smoothly so that the expansion of the depletion layer increases gradually with the applied voltage. Recently, however, there have been demands for a variable-capacitance diode device which is designed such that a wide range of variation in the depletion layer width occurs with respect to the narrow range of variation in the applied voltage and sufficient tuning capacitance is available.
The conventional variable-capacitance diode device will now be described with reference to FIG. 1 wherein the abscissa represents the depth X.sub.i from the surface of the semiconductor substrate, and the ordinate indicate the impurity concentration C on a semi-logarithmic scale. In FIG. 1, the dotted curve (10) represents a semiconductor layer of the N.sup.+ conductivity type formed through a thermal diffusion of the impurity; and the solid curve (11) indicates a semiconductor layer of the P.sup.+ conductivity type. A PN junction J is defined between the N.sup.+ and P.sup.+ type semiconductor layers. Thus, the impurity concentration in the semiconductor layer of the N.sup.+ conductivity type as indicated by the curve (12) decreases smoothly except for the PN junction J, and it is usual that the concentration curve (12) represents a concentration profile in the form of Gaussian distribution which is proximate to the curve (10). Alternatively, the arrangement is made such that the impurity concentration decreases in steps along a curve corresponding to the Gaussian distribution and the width of the depletion layer expands with an increase in the voltage applied across the PN junction. In FIG. 1, the region indicated at (13) corresponds to an epitaxial layer, and the region shown at (14) corresponds to substrate.
However, it will be seen that the profile of impurity concentration in the N.sup.+ conductivity type semiconductor layer (indicated by the curve (12)), except in the proximity to the PN junction J, is such that the following relationship holds: EQU A.sub.i &gt;A.sub.i+1
on the assumption that the impurity concentration at the highest point of the curve is A.sub.1, and those at sequential points are A.sub.2, A.sub.3, . . . , A.sub.i, A.sub.i+1, . . . respectively. The profile of impurity concentration in the region A.sub.1, A.sub.2, A.sub.3 appears approximate to the Gaussian distribution, and tends to swell out. Such swelling will be described with reference to FIG. 2 which illustrates the relationship between the applied voltage and the capacitance on semi-logarithmic scale.
As shown at (I) in FIG. 2, the relationship between the applied voltage and the capacitance varies along a curve resembling inverted S-shape corresponding to the profile of impurity concentration, instead of changing linearly from the maximum impurity concentration C.sub.max to the minimum impurity concentration C.sub.min. Thus, when the voltage range over which the capacitance is usable as tuning capacitance is wide, sufficient tuning capacitance is available, and critical problems are relatively less likely to arise. In contrast thereto, when it is desired that a variable-capacitance diode device having a similar construction to that of the prior art be operated with an applied voltage as low as 1 or 2 V, i.e., when it is attempted to make use of tuning capacitance occurring over a narrow voltage range as shown at (II) in FIG. 2, such a disadvantage that sufficient capacitance variation is not available, and thus the conventional device needs to be improved in this respect.